基本概念
chrono包括三个基本的概念:间隔(Duration),时间(Clock),时间点(Time Point),通过这三个基本概念可以很好的将时间处理抽象出来。
引用C++11的文档说明
Durations
They measure time spans, like: one minute, two hours, or ten milliseconds.
Time points
A reference to a specific point in time, like one’s birthday, today’s dawn, or when the next train passes.
Clocks
A framework that relates a time point to real physical time.
Duration
C++11 标准中的对Duration的定义
template <class Rep, class Period = ratio<1> >
class duration;A duration object expresses a time span by means of a count and a period.
使用基本的时间单元
其有六种预定义的时间单元:hours,minutes,seconds,milliseconds,microseconds,nanoseconds,如下:
minutes m1(3); //代表3分钟
minutes h1(60); //代表60分钟
//minutes 在chrono 中已经预定义
//typedef std::chrono::duration<long,std::ration<60>> minutes;
milliseconds ms(10); //代表10毫秒
milliseconds ms(1000); //代表1000毫秒即1秒
定义有含意的时间单元
在音视频系统，在编码端会设置采集或编码的帧率，比如设置为25帧。那么通过duration定义如下时间单元:
//在25帧的情况下，代表每帧的时间间隔即为40ms
typdef std::chrono::duration<long,std::ration<1,25>> FrameRate;
Clock
如定义所说是将时间点转换为系统时间，其包含三种类型及三种clock类型的作用说明如下:
system_clock
system_clock is useful when you need to correlate the time with a known epoch so you can convert it to a calendar time. Note the specific functions in the system_clock class
system_clock即为系统时间，如下代码是输出epoch value
std::chrono::system_clock::time_point epoch;
std::time_t tmEpoch = std::chrono::system_clock::to_time_t(epoch);
std::cout<<"value is "<<ctime(&now)<<std::endl;
//输出 Thu Jan 1 08:00:00 1970//获取当前时间
system_clock::now();steady_clock
steady_clock is useful when you need to wait for a specific amount of time. steady_clock time can not be reset. As other steady clocks, it is usually based on the processor tick.
steady_clock其实是tick时间，steady_clock::now的值绝不会小于上一次调用steady_clock::now时的值，即使改了系统时间，也会保证steady_clock::now产生正确的值。
high_resolution_clock
When available, high_resolution_clock is usually more expensive than the other system-wide clocks, so they are used only when the provided resolution is required to the application.
high_resolution_clock::now 返回系统支持最小单位的tick period
三种clock的精度
在每个clock类型都一个period的成员变量,它是一个ration类型(比率),以一秒为分子,精度为分母,可以通过打印period的den成员值来确定clock的精度,如下:
std::cout << system_clock::period::den<<std::endl;//输出100000000
std::cout << high_resolution_clock::period::den<<std::endl;//输出1000000000
std::cout << steady_clock::period::den << std::endl;//输出1000000000
std::cout << (double)high_resolution_clock::period::num/(double)high_resolution_clock::period::den<<std::endl;
//1e-09
std::cout << (double)system_clock::period::num/(double)system_clock::period::den<<std::endl;
//1e-07
std::cout << (double)steady_clock::period::num/(double)steady_clock::period::den << std::endl;
//1e-09上面的代码是在vs2015上编译测试的结果,system_clock精度为100纳秒,high_resolution_clock精度为1e-09(应该是基于tick),steady_clock精度为1e-09(应该是基于tick)。精度取决于系统的支持,可能在linux下精度会不同了。
Time points
顾名思义代表一个时间点,结合duration,clock可以进行时间换算。将时间点加减一个时间间隔还是一个时间点，再通过clock将时间点换算成具体时间，如下代码:
using namespace std::chrono;
//定义了一个时间点，代表从epoch后的一秒钟
time_point <system_clock, duration<int>> tp_seconds(duration<int>(1));
//将time_point转换为clock
system_clock::time_point tp(tp_seconds);
//格式化输出
std::cout << "1 second since system_clock epoch = ";
std::cout << tp.time_since_epoch().count();
std::cout << " system_clock periods." << std::endl;
std::time_t tt = system_clock::to_time_t(tp);
std::cout << "time_point tp is: " << ctime(&tt);
应用
基于steady_clock的定时器
在我前面的一篇文章中介绍了,用boost asio deadline_timer实现的定时器,deadline_timer是基于系统时间的,所以存在一个问题,当人为的改动系统时间时,定时器就失效了。其实在asio中提供一个功能跟deadline_timer相同的定时器 basic_waitable_timer,其是为兼容std的chrono库而实现的,在asio中通过basic_waitable_timer预定义了三种类型定时器:boost::asio::steady_timer,boost::asio::system_timer,boost::asio::high_resolution_timer,这三种定时器就是使用std chrono库中的stready clock,system clock,high resolution clock来实现内部的时间计算。所以可以使用boost::asio::steady_timer去解决上述问题，因为steady clock是基于tick的，人为改动系统时间比不会导致定时器失效，代码如下:
#ifndef ASIO_TIMER_H
#define ASIO_TIMER_H
#include <map>
#include <vector>
#include <mutex>
#include <boost/asio/io_service.hpp>
#include <boost/asio/steady_timer.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/asio/placeholders.hpp>
typedef void(*ProcessFun)(void*);
//以steady_timer替代原来的deadline_timer
typedef boost::shared_ptr < boost::asio::steady_timer> pSteadyTimer;
struct STimerUnit
{
int id;
int seconds;
pSteadyTimer t;
ProcessFun fun;
};
typedef boost::shared_ptr<STimerUnit> TimerUnitPtr;
class CTimer
{
public:
CTimer():m_ioWork(m_ioService),m_lID(0)
{
}
public:
//添加一个定时业务,f为业务处理函数,arg为自定义参数，seconds为超时秒数
//返回生成的ID
int AddTimerUnit(ProcessFun f, void* arg, int seconds);
//每intervalSeconds秒数执行一次 f函数
int AddTimerIntervalUnit(ProcessFun f, void *arg, int intervalSeconds);
//删除指定定时器
void RemoveTimerUnit(int id);
bool TimerisValid(int id);
void Run();
private:
void TimerProcess(int id, void* arg, bool isIntervalTimer,const boost::system::error_code& e);
private:
std::map<int, TimerUnitPtr> m_mapTimerUnits;
private:
boost::asio::io_service m_ioService;
boost::asio::io_service::work m_ioWork;
private:
std::mutex m_mutexTimerUnit;
private:
//分配timer id
std::vector<int> m_vecTimerUnitIDs;
unsigned long long m_lID;
};
#endif
#include "log.h"
#include "asiotimer.h"
int CTimer::AddTimerUnit(ProcessFun f, void* arg, int seconds)
{
TimerUnitPtr s(new STimerUnit);
s->seconds = seconds;
s->t.reset(new boost::asio::deadline_timer(m_ioService, boost::posix_time::seconds(seconds)));
s->fun = f;
{
std::lock_guard<std::mutex> lock(m_mutexTimerUnit);
m_mapTimerUnits.insert(std::make_pair(++m_lID, s));
s->t->async_wait(boost::bind(&CTimer::TimerProcess, this, m_lID, arg, false,boost::asio::placeholders::error));
return m_lID;
}
}
int CTimer::AddTimerIntervalUnit(ProcessFun f, void *arg, int intervalSeconds)
{
TimerUnitPtr s(new STimerUnit);
s->seconds = intervalSeconds;
s->t.reset(new boost::asio::deadline_timer(m_ioService, boost::posix_time::seconds(intervalSeconds)));
s->fun = f;
{
std::lock_guard<std::mutex> lock(m_mutexTimerUnit);
m_mapTimerUnits.insert(std::make_pair(++m_lID, s));
s->t->async_wait(boost::bind(&CTimer::TimerProcess, this, m_lID, arg, true, boost::asio::placeholders::error));
return m_lID;
}
}
void CTimer::RemoveTimerUnit(int id)
{
std::lock_guard<std::mutex> lock(m_mutexTimerUnit);
std::map<int, TimerUnitPtr>::iterator It = m_mapTimerUnits.find(id);
if (It != m_mapTimerUnits.end())
{
It->second->t->cancel();
m_mapTimerUnits.erase(It);
return;
}
}
bool CTimer::TimerisValid(int id)
{
std::lock_guard<std::mutex> lock(m_mutexTimerUnit);
std::map<int, TimerUnitPtr>::iterator It = m_mapTimerUnits.find(id);
if (It != m_mapTimerUnits.end())
{
return true;
}
return false;
}
void CTimer::Run()
{
m_ioService.run();
}
void CTimer::TimerProcess(int id, void* arg, bool isIntervalTimer, const boost::system::error_code& e)
{
if (e == boost::asio::error::operation_aborted)
{
return;
}
TimerUnitPtr pTimerUnit;
{
std::lock_guard<std::mutex> lock(m_mutexTimerUnit);
std::map<int, TimerUnitPtr>::iterator It = m_mapTimerUnits.find(id);
if (It != m_mapTimerUnits.end())
{
pTimerUnit = It->second;
if (!isIntervalTimer)
{
m_mapTimerUnits.erase(It);
}
}
LOG_INFO << "=========>mapTimerUnits size " << m_mapTimerUnits.size() << std::endl;
}
if (pTimerUnit)
{
pTimerUnit->fun(arg);
if (isIntervalTimer)
{
pTimerUnit->t->expires_at(pTimerUnit->t->expires_at() + boost::posix_time::seconds(pTimerUnit->seconds));
pTimerUnit->t->async_wait(boost::bind(&CTimer::TimerProcess, this, id, arg, true, boost::asio::placeholders::error));
}
}
else
{
LOG_INFO << "TimerUnit pointer is NULL" << std::endl;
}
} 通过Duration实现准确帧率
音视频系统中，在编码或采集或渲染时，通常需要以固定帧率去进行。比如以25帧进行采集，那么可通过如下代码来实现准确的时间间隔
//定义了25帧时的时间间隔即为40ms
typedef std::chrono::duration<long,std::ration<1,25>> FrameRate;
auto StartTime = std::chrono::steady_time::now();
auto EndTime = std::chrono::steady_time::now() + FrameRate(1);
采集操作
std::this_thread::sleep_until(EndTime);参考资料
https://www.boost.org/doc/libs/1_67_0/doc/html/chrono/users_guide.html#chrono.users_guide.tutorial.duration.can_durations_overflow_
http://www.cplusplus.com/reference/chrono/
https://stackoverflow.com/questions/26501936/difference-between-deadline-timerand-waitable-timer-in-boost-asio
https://stackoverflow.com/questions/16354727/boost-deadline-timer-minimal-example-should-i-substitute-sleep/16364002#
https://stackoverflow.com/questions/20375140/c11-threads-sleep-for-a-remaining-time
以上。